1. Field of the Invention
The invention relates to a method and device for the cooking and vacuum packing of mussels by the use of microwaves, such that said mussels can be stored as chilled or frozen goods.
2. Background Information
A dish of mussels attains its best quality when prepared from fresh, live mussels. Such mussels still retain seawater within their shells, which yields an delicious liquor when cooked. After being out of the water for a time. Mussels tend to open and the trapped water escapes, whereupon the mussels dry out and eventually die.
The smaller-sized young mussels die more rapidly in this way than large, old mussels, which remain closed for a longer time when kept out of the water.
Because of their short survival time out of water, these young mussels are never seen on sale; the fresh mussel market has to make do with older, large and often tough mussels. If it were possible, the consumer would prefer smaller young mussels with tenderer meat and a larger proportion of meat.
Mussels, like other shellfish, cannot be frozen fresh and live without first undergoing heat treatment, because when frozen fresh the meat does not coagulate satisfactorily during cooking and on final preparation the result is meat that sticks to the shell and is of poor consistency. For satisfactory coagulation of the mussel meat during cooking, the heating phase must be rapid and the mussel should be cooked through within three minutes. To achieve such rapid heating, mussels are cooked industrially in an autoclave by means of steam which is fed rapidly at approximately 125 degrees Celsius. Equally rapid coagulation can be achieved by microwave heating, but when cooked in this way the mussels xe2x80x9cexplode,xe2x80x9d whereupon the juice escapes and the mussel meat dries out.
In order to avoid oxidation (rancidity) of the fat that they contain, it is usual to vacuum pack shellfish with an inert gas, e.g. carbon dioxide. Mussels can be vacuum packed if they have strong shells, i.e. older mussels whose shells have thickened. Young mussels have thin shells which break very easily when the bag is evacuated, and these mussels are therefore very difficult to pack in vacuum bags without excessive shell breakage.
If a vacuum-packed bag of mussels is heated in a microwave oven, the bag expands and bursts as the water contained in the mussels is vaporised. For this reason such bags are generally heated in water, in which case heating does not take place as quickly is desirable in order for the mussel meat to coagulate in a satisfactory manner (see above). To achieve sufficiently rapid cooking of vacuum packed mussels a retort cooker with overpressure facility is used (Patent No EP 0 721 743 Al Turner Nelson NZ). The entire process including come-up time and cooling in the cooker takes approximately 30 minutes and the cooking process cannot be made continuous. By comparison, in the present method, evacuation and cooling take place in the same operation in less than five minutes in a process which can be made continuous in a microwave tunnel.
There exist special packages and containers for heating by microwave energy: EP Patent No 0 174 188 relates to a gas-tight plastic can having a valve which is opened once to release overpressure as the can is heated. This patent relates to the sealing of the vent opening, which is designed to open spontaneously during microwave heating. U.S. Pat. No 5,750,967 relates to a plastic container for the storage and microwave heating of foodstuffs, which container can be reused. A valve is provided on the upper side of the pack to release overpressure arising during cooking and to admit air during cooling, so that the pack does not deform. Furthermore, UK Patent No 1 550 000 (Unilever) relates to a packaging which is vented in a special way during heating and cooling so that vacuum does not arise inside the packaging.
One method of producing frozen mussels in their shells is to pack said mussels prior to cooking into a large (approx. 40 kg) container which is then submerged in hot water (EP 0 094 362 Al Lars Goran Carlsson). After heating, the container is frozen with the mussels inside. When the mussels are frozen, the container is opened and the mussels are removed and shaken apart in their frozen state so that they are single. This process too requires mussels with thick shells so that they do not break during handling. The final product is single cooked mussels with shells. In this patented process, once again, the cooking is too slow (see above), the final product is not pasteurised, and the process requires mussels with thick shells, i.e. old, larger mussels.
The purpose of the invention is to make possible the use of young, thin-shelled mussels which according to most connoisseurs have the best consistency, size and taste, for a pasteurised or frozen product which can be stored for a considerable time prior to its final use. The same invention is also suitable for the packaging of other liquid or solid foodstuffs industrially or for home use in order to obtain a pasteurised or frozen product.
In accordance with the purpose of the invention, as one aspect of the invention, a method is provided for the cooking and vacuum packing of food, such as mussels, comprising heating food in a container, wherein the container is provided with a venting opening for the release of overpressure during boiling. Microwave energy may then be used to heat and cook the food, resulting in steam being formed inside the container. Excess steam and air may then exit through the venting opening, which is sealed manually or automatically when the input of microwave energy is ceased. As the container cools, steam therein condenses, whereupon a vacuum arises inside the container. Due to the use of a flexible container, the container molds itself to its contents as the vacuum arises in the container.
In accordance with another aspect of the invention, a device for cooking and vacuum packing food, such as mussels, is provided which comprises a flexible container made of material which can withstand microwave heating and frozen storage and is tough enough to be suitable for vacuum packing. A venting opening is provided on the top side of the container through which air and steam can escape during boiling, and which may be sealed manually or automatically by means of self-adhesive tape as soon as boiling ceases. The flexible container molds itself to the food when a vacuum arises in the container as steam condenses due to cooling of the container following cessation of microwave heating.